Transcriptional regulators play pivotal roles in many physiological and pathological processes such as cell proliferation and differentiation, as well as oncogenic transformation. A main interest of my lab is to understand the biology and mechanism of action of Yin Yang 1 (YY1), a C2H2-type zinc finger transcription factor that can both repress and activate transcription. YY1 has been implicated in cell growth and transformation, but such a function has not been demonstrated experimentally, and thus the cellular function of YY1 has remained speculative. In the past funding period, we have made the exciting discovery that YY1 plays an essential role in p53 homeostasis, which impacts on cell growth and apoptosis. At least part of this regulation appears to be independent of YY1 transcriptional role, thus uncovering a novel, non-transcription-based mechanism by which YY1 regulates cell physiology in vivo. These findings have filled an important gap in our understanding of YY1 and also have significant implications for p53 regulation in vivo. A main focus of this application is to understand the mechanisms by which YY1 regulates p53 stability, which is controlled primarily by Mdm2-mediated ubiquitination and proteasome degradation. We will test several mutually non-exclusive models, including YY1 regulation of mdm2-p53 via physical interactions, YY1 cooperation with Mdm2 to stimulate p53 ubiquitination, as well as the possibility of YY1 regulation of deubiquitinases (Aims 1 and 2). Multiple lines of evidence, including the YY1-p53 connection, predict a function for YY1 in cell proliferation and oncogenic transformation, which will be investigated using the YY1-deficient MEF cells that we have developed recently (Aim 3). These cells also make it possible to identify, at the genome-wide level, YY1 target genes in vivo (microarray) (Aim 4). Knowledge of bona fide YY1 target genes is likely to shed light on potential molecular basis of YY1 functions, and will allow us to test, at the genome-wide level, the hypothesis that YY1 activates and represses transcription by inducing differential histone modifications (Aim 4). Taken together, the proposed studies will not only provide molecular insights into YY1 but also shed light on general mechanisms that control p53 homeostasis, cell proliferation, oncogenic transformation and differentiation.